In a 3GPP LTE-Advanced system (LTE is the abbreviation of Long Term Evolution, and is an evolution system of the third generation mobile communication system, an LTE-Advanced system is an advanced version of LTE system, abbreviated as LTE-A), the peak rate is greatly improved compared with that of the LTE system. The downlink peak rate is required to reach 1 Gbps, and the uplink peak rate is required to reach 500 Mbps. At the same time, the LTE-A system is required to have a very good compatibility with the LTE system. Based on needs of improving the peak rate, the compatibility with the LTE system and making full use of spectrum resources, the LTE-A system introduces a carrier aggregation (CA) technology.
The carrier aggregation technology means that the uplink and downlink in a cell respectively contain multiple component carriers rather than the mode of having only a carrier in the LTE and earlier wireless communication systems. In a CA system, each component carrier can be continuous or discontinuous. In order to be compatible with the LTE system, the maximum bandwidth of each component carrier is 20 MHz, and the bandwidths of different component carriers can be identical or different.
With respect to a transmission technology of single carrier frequency division multiple access (SC-FDMA), in order to maintain orthogonality between uplink signals of different user equipments (UE), it should be ensured that receiving clocks of the signals of various UEs at the receiver side are the same. The base station instructs the UE to adopt different uplink timing advance through uplink time synchronization adjustment signaling, to make up propagation delay of the uplink signal, so that signals of the UEs located at different locations in the cell can reach the base station substantially at the same time.
The LTE uses timer-based uplink synchronization maintenance mechanism, a configurable timing alignment timer (TAT) and an uplink timing adjustment command (TA Command, TAC) are maintained for each UE. After the UE establishes a connection, the TAT is maintained for the UE and the base station. Before the TAT expires, the base station sends to the UE an uplink timing adjustment command. If the UE receives the uplink timing adjustment command, then the UE adjusts the uplink transmission time according to TA (Timing Advance) parameters; if the UE does not receives the uplink timing adjustment command before the timer expires, then the UE may consider that uplink transmission loses synchronization. The purpose of the uplink synchronization maintenance is to ensure that through reasonably configuration and maintenance of the TAT, the UE and the base station is enabled to try their best to make the TAT not expire so as to keep the uplink synchronization between the UE and the base station, and is also to try to send less TACs and reduce control signaling on the uplink and the downlink for uplink synchronization.